On confirmation bias

If a man objects to truths that are all too evident, it is no easy task finding arguments that will change his mind. This is proof neither of his own strength nor of his teacher's weakness. When someone caught in an argument hardens to stone, there is just no more reasoning with them.

Epictetus, Discourses I.5.1-2 (page 15 of this edition) (original Greek, with alternate translations at the link)